There's a difference between saying something is inspired by X vs saying something is copying X.

OpenTTD is a great example because it has its own selection of art assets that look like the original, but aren't copying the original art. The open art assets are inspired by the original. But OpenTTD doesn't come with the original assets nor can it make exact copies of the original assets because that would be copying the original.

I'm not against being inspired by something, Allumeria dev has been pretty open that the game is inspired by Minecraft. But the people I've been arguing with here have the opinion (at least it seems to be that way) that Allumeria is copying Minecraft. It is a bad precedent to say Allumeria is copying Minecraft (not just for Allumeria but a lot of Minecraft "clones" because the blocky style limits quite a lot of what you can artistically do) and it would be a bad precedent to say openTTD assets were copying original TTD assets, because that could be grounds for copyright which could lead to openTTD assets having to be scrapped.